RE: [Openexr-devel] HDRI usage questions...

[Thomas Driemeyer]

On Sat, 12 Jul 2003, Nick Porcino wrote:
> As a result when we merge the images together, we are finding that
> there are very obvious seams since the camera matrices have been set up
> differently to generate the various views.

Well, specularity is computed in object space, but the exact point on
the surface where it is done depends on the eye ray direction, and hence
on the camera space transformation. You'll see that especially on very
small or very thin highlights. That's a normal aliasing effect, but with
a twist: normally aliasing is handled well by adaptive oversampling, but
with HDR and pixel energies far out of the 0..1 range a tiny change in
the sample position can have a huge effect on the value stored in the HDR
image.

Or in other words: if a subsample hits the super-bright highlight, its
energy is huge, otherwise it's small. Any pixel with at least one huge
subsample will be huge. If a camera space change makes a huge subsample
drift into a previously low-energy neighboring pixel, you see the change.

I don't see an easy solution to that. Perhaps a custom energy-conserving
despeckle filter, or a wider mental ray sample filter such as clipped
Lanczos, would help? It would have to operate on samples, not pixels.
(Hm, I wonder whether irregular sampling grids instead of regular pixel
grids would make a good feature extension for OpenEXR... mental ray can
write sample files, but it uses a nonstandard compressed format.)

I assume you are not talking about simple terminator effects at triangle
edges caused by insufficient tessellation. Those are visible without HDR.

Thomas

--
Thomas Driemeyer <address@hidden> +49 30 3159970    mental images, Berlin